This application claims the priority of German application 197 21 852.0, filed May 26, 1997 in Germany, the disclosure of which is expressly incorporated by reference herein.
Gear transmissions that operate with zero play are already known in which one gear of a pair of gears that mesh with one another consists of two tooth crowns, one of which transmits the torque while the other compensates for play, said crowns being biased against one another by a spring in the circumferential direction, i.e. movably connected with one another. German Utility Model 74 00 276 describes such a gear transmission, with a gear composed of two gear elements biased against one another.
Previously known engine and transmission designs however suffer from the problem that the housings have thermal expansion characteristics that differ from those of the shafts, bearings, and gears that are employed. This is especially true when steel gears are used in aluminum housings.
Even when gears with very little tooth flank play at ambient temperatures of about 20.degree. C. are deliberately installed, the change in axial spacing that takes place at higher operating temperatures results in a significant increase in tooth flank play. As a result, a clearly perceptible rattling noise can develop.
For this reason, the tooth flank play of a pair of gears that mesh with one another is compensated by an additional gear element mounted coaxially to a gear, said gear element being biased by one or more springs and urged movably in the circumferential direction against the gear.
This has the disadvantage that usually only the idle noise of the gears that are in mesh is influenced in a positive manner. Effective compensation can occur in only one load direction. When loads change, the spring is subjected to excess pressure and thus the rear tooth flank comes in contact. During coasting, this usually means that a rattling noise results. It is also disadvantageous that the spring force produces increased friction on the tooth flanks, which usually is manifested by a squeaking sound.
Therefore a goal of the invention is to provide a transmission gear that compensates for tooth flank play in a pair of gears in a specific manner regardless of the spring forces and thus independently of the torque being transmitted at a given moment by the pair of gears.
This goal is achieved according to preferred embodiments of the invention by providing a transmission gear composed of at least two gear elements provided with teeth on their circumference, said elements being displaceable coaxially in the circumferential direction, with their teeth offset with respect to one another, and connected shapewise with one another by at least one connecting member in such fashion that for engagement of a meshing gear, the existing tooth flank play is reduced or eliminated, wherein the connecting member connects the gear elements so that they are not displaceable in the circumferential direction and, by its thermal expansion, causes shifting of the gear elements with respect to one another in the circumferential direction.
According to the invention, a transmission gear assembly includes of at least. two gear elements provided with teeth on their circumference, said gear elements being displaceable coaxially in the circumferential direction and having teeth that are offset with respect to one another, and connected shapewise with one another by at least one connecting member in such fashion that the existing tooth flank play is reduced or eliminated in order to engage a meshing gear, with the connecting member connecting the gear elements nondisplaceably with one another in the circumferential direction and, by virtue of its thermal expansion, causing shifting of the gear elements with respect to one another in the circumferential direction.
When the gear elements that are mounted displaceably with respect to one another are firmly connected with one another by a connecting member, this has the advantage that their shifting relative to one another is based on a deliberate utilization of different coefficients of thermal expansion of the materials used for the gear elements and the connecting member. In this way, the tooth flank play that results from differences in thermal expansion between the gears and the transmission housing can be compensated extremely well. The connecting member between the gear elements can consist either of a material that has a degree of thermal expansion that is deliberately greater or deliberately smaller than that of the gear elements. By careful positioning and length of the connecting member, complete tooth flank play compensation or clearly reduced tooth flank play can be achieved over the entire operating temperature range from less than -40.degree. C. to far above 200.degree. C.
Advantageously, the gear elements are not tensioned against the gear with which they mesh, but only the play between the toothed flanks is compensated. For this reason, the frictional losses are significantly less than when tooth flank play compensation is performed by spring force. This also has a positive effect on efficiency and wear in the transmission.
In addition, tooth flank play compensation according to the invention is ensured in both directions of rotation of the gear pair. As a result, the rattling noises produced during coasting as well as nonuniformity of rotation are reduced.
In preferred embodiments of the invention, the connecting member is a pull-push rod. This has the advantage that the twist angle between the two gear elements at a given temperature can be determined in advance in simple fashion.
Another advantageous feature of preferred embodiments of the invention is that the connecting member is mounted like a chord between the two gear elements and is connected shapewise at each end with a respective gear element.
A transmission gear constructed according to the invention is also advantageously easy to manufacture.